Distributed thermal negative lensing in high power gas lasers, due to hot gas (or plasma) in laser optical cavities, is a well known phenomenon. As the active length of a sealed-off (or slow flow) CO.sub.2 laser is increased above six to eight meters, the negative lensing effect, caused by the radial temperature gradient and hence the density gradient in the mixture of gases serving as the active medium, cannot be neglected. This causes the laser beam diameter to become larger than would be expected without such negative lensing. Negative thermal lensing also occurs in high power gas lasers other than high power CO.sub.2 lasers.
Throughout this specification, the phrase "high power" laser system will denote a laser system producing a laser beam having power greater than 200 watts. In a slow-flow CO.sub.2 laser, it would be desirable to increase the optical cavity's length to increase the output beam power above 750 watts (to as much as 1.6 kW or more). However, the negative thermal lensing phenomenon limits the efficiency and maximum output power of conventional slow-flow CO.sub.2 laser systems.
The negative lensing phenomenon in high power CO.sub.2 laser systems can sometimes be ignored in the special case that smooth-bore plasma tubes are employed in the optical cavities of such lasers. Such smooth-bore plasma tubes function as waveguides confining both fundamental and higher order modes with low propagation losses. However, it is often desirable to suppress such higher order modes.
For example, the high power, folded, carbon dioxide gas laser system of U.S. Pat. No. 4,500,996, issued on Feb. 19, 1985 to Sasnett et.al. and assigned to Coherent, Inc. (the assignee of the present application), employs rippled plasma tubes (having periodic reduced diameter sections) in the system's folded optical cavity to suppress waveguiding. By employing rippled plasma tubes, the losses for the higher order modes become sufficiently large that these higher order modes do not lase. Thus, the output laser beam emerging from the system of U.S. Pat. No. 4,500,996 includes basically only the fundamental TEM.sub.oo mode.
Normal diffraction causes the beam inside the optical cavity to change diameter throughout the length of the cavity. For CO.sub.2 lasers longer than a few meters this makes it necessary to use rippled plasma tubes of differing diameters to confine the beam and the excited gas in the same volume, and thereby achieve full output lasers longer than 6 to 8 meters, the negative thermal lensing aggravates this problem of having different beam diameters at different locations along the optical cavity. It would be desirable to improve high power, folded cavity, fundamental mode gas laser systems (such as the system described in U.S. Pat. No. 4,500,996) so that the beam diameter is substantially constant along the entire length of the optical cavity, so that plasma tubes of equal diameter may be employed without reducing the system's output power and efficiency. It would also be desirable to be able to choose and control this beam diameter as part of the laser design rather than to have to accept the diameter dictated by end mirror curvature, diffraction and negative thermal lens effects.
In principle these goals could be achieved by providing periodic refocusing of the beam either with one or more lenses of positive focal length distributed throughout the optical cavity or by replacing conventional flat corner mirrors with off-axis parabolic mirrors to provide refocusing. In practice neither of these approaches is acceptable. The use of one or more lenses inside the optical cavity is precluded by the dependence of the focal length of the lens(es) on the power absorbed in the lens(es) and hence on the circulating power inside the cavity. The use of off-axis parabolic surfaces is precluded by their high cost.